1. Field of the Invention
The present invention relates to method for creating dot arrangements or threshold matrices, an image processing apparatus, and a recording medium, and more particularly to a method for creating dot arrangements or threshold matrices that are used in processing when forming an image by an image processing apparatus, an image processing apparatus that uses that method, and a storage medium.
2. Description of the Related Art
Image formation apparatuses that form images using dots on a recording medium are often used as apparatuses that output images that were processed by a personal computer or images that were taken by a digital camera or the like. Of such image formation apparatuses, a method of forming images on a recording medium by causing a recording material to adhere to the recording medium is widely used, and as a representative example, an inkjet recording method is known. In order to improve the recording speed and increase the image quality, an image formation apparatus that employs the inkjet recording method comprises a nozzle group in which plural ink ejection openings (nozzles) that are capable of ejecting ink of the same color and same density are collectively arranged. Furthermore, in order to improve the image quality, a nozzle group that is capable of ejecting ink having the same color but different density, or a nozzle group that is capable of discharging ink of the same color and same density by changing the amount of ink discharged in stages may also be provided.
This kind of image formation apparatus forms a final image by different nozzle groups printing plural times in the same main scanning printing area of a specified recording medium. When doing this, it is known that the image quality of the image that is finally formed is affected by the differences in the overlapping images by each of the nozzle groups (in other words dot arrangements of the ink). In an actual image formation apparatus it is difficult to eliminate the change in physical registration such as the conveyance amount of the recording medium or the position displacement of nozzles, so shifting of the impact position of the ink from each nozzle groups with respect to the target position cannot be avoided. Shifting in the impact position of the ink from each of the nozzle groups becomes the cause of lightness fluctuation in which the density of the image formed changes due to whether or not there is shifting of the impact position, uneven density, poor image quality due to graininess and the like. Therefore, this leads to the intended image not being formed when the nozzle group overlaps printing plural times.
In regard to this, technology is disclosed that reduces degradation of the formed image by using a dot arrangement for the dot arrangement of the ink formed by each of the nozzle groups that is not easily affected even when there is shifting of the impact position (in other words, a dot arrangement that is robust against a shift in position). Japanese Patent Laid-Open No. 2010-274656 discloses, for example, technology for creating a threshold matrix so that the threshold matrix that sets the dot arrangement of the nozzle groups creates dot arrangements having high dispersion. Japanese Patent Laid-Open No. 2008-188805 discloses technology, for example, that determines a dot arrangement so that the phase difference of the dot arrangements of each nozzle group satisfies a low-frequency reverse phase frequency characteristic.
However, the technology that is disclosed in Japanese Patent Laid-Open No. 2010-274656 does not take into consideration shifting in the sub-scanning direction (conveyance direction of the recording medium). Moreover, even when there is high dispersion of the dot arrangements that are made by the nozzle groups, the dots of the image that is finally formed is not necessarily robust against position shift. Furthermore, the technology that is disclosed in Japanese Patent Laid-Open No. 2008-188805 uses the concept that a low-frequency reverse phase characteristic will more widely cover the robustness against position shift, however, the control in high-frequency areas is insufficient, and there is a possibility that there will be no improvement in graininess.